The present invention relates to a device for engaging the four wheel drive transmission of a vehicle such as, for example, an agricultural tractor or an off-the-road vehicle.
It is known that in the majority of vehicles provided with a device of the type indicated above the drive wheels are the rear wheels whilst the front wheels can also become driven by effecting a mechanical connection between the rear wheel drive transmission shaft and that of the front wheels. This mechanical connection is achieved by means of an hydraulic or mechanical clutch which can be actuated with a manual control.
In an agricultural tractor provided with a conventional clutch for the four wheel drive transmission, when the clutch is engaged the front wheels are normally driven slightly faster, by about 2%, than the rear wheels. This speed differential is caused by the geometry of the transmission members fitted to the front wheels. In the field, that is when the tractor is travelling over agricultural ground, because of the low speed at which the tractor normally runs and because of the fact that the agricultural ground offers a low adhesion to the tyres, it is established that with the four wheel drive transmission engaged all the wheels may slip even by up to 10-15% with respect to the speed imposed by the engine of the tractor so that the difference in the speed between the front wheels and the rear wheels is not noticed. On asphalted roads, because the speed is normally greater than on agricultural ground, because there is a greater adhesion between the road surface and the tyres, and because in tractors the majority of the weight is supported by the rear wheels (and therefore also the traction force on the ground caused by the rear wheels is greater than that between the front wheels and the ground) when the four wheel drive transmission is engaged there is a considerable slippage of the front wheels with respect to the ground due to the speed differential imposed on these latter by the transmission kinematics. This causes a high wear on the front tyres so that on asphalted roads it is preferred to disengage the clutch and therefore maintain only the rear wheels as drive wheels. In bends, especially in those of a short radius, both in the field and on asphalted roads, the behaviour of the four wheel drive is negative. In fact, in bends with the four wheel drive engaged, even when unlocking the front wheel differential, the front wheel on the outside of the bend tends to travel by up to 20% faster than the rear wheels so that this front wheel slips causing a very considerable wear on the tyre. It is evident that the requirements of the tractor are to have the four wheel drive traction engaged in a field in all circumstances and to have the four wheel drive transmission engaged when running on roads having a high adhesion and/or with high speed only when the rear wheels lose adhesion and slip with respect to the ground. Currently these requirements of tractors can be met only by way of a manual control by the operator who naturally must be expert and able to sense the said requirements of the tractor.